Over-the-ear headphones are recommended instead of earbuds or on-ear headphones because they provide an immersive sound experience facilitated by their enclosed nature.
The vagus nerve has a sensory branch from the external ear, the external auditory canal, and the pinna (which is the ear skin/tissue itself). By covering the whole ear with over-ear headphones, we’re maximizing input to the vagus nerve for maximum parasympathetic function. Earbuds only bring input to the external auditory canal and eardrum.